This invention relates to microfluidic fluid control, specifically to methods of sealing microfluidic channels and of pumping and valving in microfluidic channels.
A wide range of microfluidic pumping and valving techniques have been reported. A common feature of these devices is that they are prone to leaking. Due to the scale of microfluidic devices the ratio of the perimeter of a microfluidic channel to its cross-sectional surface area is very high so that even if a channel is in some way plugged, small leaks occurring around the perimeter of the plug result in substantial fluid flow. In particular, if the seal is formed by a solid surface contacting another solid surface any gap between the surfaces, by design or by injury, creates a sizeable path for fluid flow. The microchannel application is also complicated by the requirement of providing control surfaces in the microchannel that can be manipulated from outside of the channel.
Several devices have been reported wherein a plug of sealing fluid, such as an air bubble, a plug of mercury or a plug of ferrofluid, that is immiscible in the driven fluid is used to attempt to create a blockage in a channel. As the plug more easily conforms to the shape of the channel the possibility of a gap occurring at the sealing surface, which is the surface of contact between the the sealing fluid and the channel, is reduced.
However, if a layer of driven fluid adheres to the channel wall between the channel wall and the plug of sealing fluid then that layer can act as a pathway for flow of driven fluid past the plug of sealing fluid, creating a breach of the seal. This situation can come about in several ways. The driven fluid may spontaneously replace the sealing fluid along the channel walls. It is also possible that the plug of sealing fluid is moved to a section of channel previously occupied by the driven fluid and the sealing fluid does not replace the driven fluid as the surface layer. Regardless of the mechanism by which a leakage layer forms, in a microfluidic system, such a leak around the plug of sealing fluid can permit a high flow of driven fluid.
This invention seeks to overcome this difficulty in the prior art by providing a hermetically sealing device and a method for controlling fluid flow within a channel, using a force that is generated by a device exterior to the channel.
There is therefore provided in accordance with an aspect of the invention, a method of controlling flow of a driven fluid in a channel, the channel being defined by an encircling wall, the method comprising the steps of:
sealing the channel with a sealing fluid blocking the channel at an initial position, in which the sealing fluid has a first contact angle with the encircling wall and the driven fluid has a second contact angle with the encircling wall and the first contact angle is less than the second contact angle and in which the driven fluid is immiscible in the sealing fluid; and
moving the sealing fluid in the channel by a force generated outside of the channel.
According to a further aspect of the invention, there is provided apparatus for controlling flow of a driven fluid in a channel, the apparatus comprising:
an encircling wall defining a channel;
a sealing fluid in the channel, the sealing fluid having a first contact angle with the encircling wall;
a driven fluid in the channel having an interface with the sealing fluid, the driven fluid being immiscible in the sealing fluid and having a second contact angle with the encircling wall, the first contact angle being less than the second contact angle; and
a force generator exterior to the channel, the force generator being operable to move the sealing fluid within the channel.
It is therefore an object of this invention to prevent leakage around a microfluid seal in a microchannel, by using preferential affinity of a sealing fluid for a channel wall as compared with the affinity of a driven fluid for the channel wall, and by providing an externally generated force to control the position of the sealing fluid.
It is another object of this invention to provide fluid control devices based on application of the principle of the invention.